THE SYMPATHETIC NERVOUS SYSTEM 229 



sources of the excitations. The mechanisms shown in Fig. 

 109 show how an inflammatory process or other injury of the 

 sympathetic nerves associated with these deep viscera may read- 

 ily be carried over to the related neurons of the somatic sensory 

 system. Many referred pains are undoubtedly due to similar 

 collocations of visceral and somatic sensory paths within the 

 spinal cord and brain. Since the functions of these visceral 

 nerves do not usually come into consciousness at all, the pain 

 will be referred to the peripheral area of distribution of the 

 associated somatic nerve, which has a distinct "local sign," or 

 habitual peripheral reference. 



The efferent fibers of the cerebro-spinal visceral system arise 

 from several groups of cells in the intermediate zone between 

 the dorsal and ventral gray columns of the spinal cord, and in 

 particular from an intermedio-lateral column of cells at the mar- 

 gin of the lateral column of gray matter (Fig. 56, p. 126). These 

 efferent fibers never reach their peripheral terminations directly. 

 They always end in some sympathetic ganglion, either of the 

 vertebral ganglionic trunk or one of the peripheral sympathetic 

 ganglia. Here there is a synapse, and a second neuron of the 

 sympathetic ganglion in question takes up the nervous impulse 

 and transmits it to its termination in some unstriated visceral 

 muscle or gland. The efferent fiber arising from a cell body 

 within the spinal cord is termed the preganglionic fiber, and the 

 peripheral fiber arising from a neuron of the sympathetic gang- 

 lion is the postganglionic fiber. The former is usually a small 

 myelinated fiber; the latter is usually unmyelinated. The pre- 

 ceding description is applicable to the visceral nervous system 

 in the trunk region of the body. In the head the connections 

 of the nerves of this type are much more complex. 



Langley and others have shown that what is here termed the 

 general cerebro-spinal visceral system is related to four distinct 

 regions of the central nervous system, as illustrated by Fig. 111. 1 



1 Langley calls the entire sympathetic system the autonomic system, and 

 limits the application of the term "sympathetic" to what is here called the 

 thoracic-lumbar sympathetic. There is no adequate ground for his belief 

 that the latter is genetically different from the other parts of the cerebro- 

 spinal visceral apparatus, though its physiological characteristics are very 

 distinctive. Many of the viscera have a double innervation through the sym- 

 pathetic, one set of fibers coming from the midbrain, bulbar, or sacral sym- 

 pathetic ganglia and an antagonistic set coming from the thoracic-lumbar 

 sympathetic ganglia. 



